In recent years, from a social requirement of a lower fuel consumption and a less exhaust emission, an electric vehicle and a hybrid vehicle, each of which is mounted with an AC motor as a power source of a vehicle, is drawing attention. For example, in some of the hybrid vehicles, a DC power source made of a secondary battery or the like and an AC motor are connected to each other via an electric power conversion unit constructed of an inverter and the like, and a DC voltage of the DC power source is converted into an AC voltage by the inverter to thereby drive the AC motor.
In the control device of the AC motor mounted in the hybrid vehicle and the electric vehicle has been known the following technology (refer to, for example, a patent document 1): that is, a current sensor for sensing a phase current is provided only in one phase; and a current estimated value estimated on the basis of a current sensed value of the one phase is fed back, whereby a current flowing through an AC motor is controlled. Since the current sensor is provided only in the one phase, the number of the current sensor is reduced and the size of a portion near a three-phase output terminal of an inverter is reduced and the cost of a control system of the AC motor is reduced.
In the technique disclosed in the patent document 1, on the basis of a current sensed value of one phase (for example, U phase), which is sensed by the current sensor, and a d axis current command value and a q axis current command value and an electric angle of an AC motor, the current estimated values of the other two phases (for example, V phase and W phase) are calculated. Specifically, an angle (θ), which is formed by a rotator of the AC motor and a U phase axis of a stator, is added to a command current phase angle (α), which is acquired from a d axis current command value id* and a q axis current command value iq* to thereby find a U phase current phase angle θ′ (=θ+α). Then, a current amplitude Ia is calculated by the following formula (91) by the use of the U phase current phase angle θ′ and a U phase current sensed value Iu. Then, a sin value at an electric angle shifted by ±120 [°] from the U phase current phase angle θ′ is multiplied by the current amplitude Ia to thereby calculate the current estimated values Iv, Iw of the two other phases by the following formulas (92), (93).Ia=Iu/[√(⅓)×{−sin(θ′)}]  (91)Iv=√(⅓)×Ia×{−sin(θ′+120°)}  (92)Iw=(⅓)×Ia×{−sin(θ′+240°)}  (93)
Then, the current sensed value Iu of one phase and the current estimated values Iv, Iw of the other two phases are dq transformed to thereby calculate a d axis current estimated value Id and a q axis current estimated value Iq. Then, the current flowing through the AC motor is controlled by a current feedback control mode of feeding back the d axis current estimated value Id and the q axis current estimated value Iq to a d axis current command value Id* and a q axis current command value Iq*.
In the technique of the patent document 1, when the U phase current phase angle θ′=0 [°] and sin(θ′)=0, in the calculation of the current amplitude Ia by the formula (91), Iu is divided by 0, that is, “zero division” is caused and hence the current amplitude Ia cannot be calculated correctly. For this reason, the current estimated values Iv, Iw of the other two phases cannot be calculated correctly. However, the patent document 1 never refers to measures against this “zero division”. Further, when the current sensed value Iu=0 [A], from the formulas (92) and (93), the current estimated values Iv, Iw of the other two phases are calculated as follows: Iv=0 [A] and Iw=0 [A]. Hence, there is a possibility that the control of the AC motor could not be performed.
Also in a technique other than the patent document 1, in the case where a variable to become 0 in a specified phase or at a specified timing is included, there is a possibility that a correct calculation could be impaired by “the zero division” of dividing something by 0 or “the zero multiplication” of multiplying something by 0 and hence the current estimated value could be varied.    [Patent document 1] JP-A 2004-159391